drm/amd/display: Guard vblank wq flush with DCN guards

[Why]
Compilation of the workqueue fails if not building with the DCN config
option set.

[How]
Guard calls to the flush with the DCN config option to fix the build.

Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>